The battle to contain devastating wildfires across Colorado reached a new and more aggressive stage Monday as C-130 military tankers joined the fight to bombard the Waldo Canyon blaze with thousands of gallons of fire retardant.

Four of the C-130 aircraft, two from the 302nd Airlift Wing of the U.S. Air Force Reserve at Peterson Air Force Base and two from the 153rd Airlift Wing of the Wyoming Air National Guard in Cheyenne, took to the skies with 3,000 gallons of fire retardant in each of their bellies.

The planes can drop their huge payloads in less than five seconds and can be refilled in 15 minutes.

There are only eight such C-130s, plus one spare, in all of the United States that can be fitted with the special firefighting equipment, and half the fleet is now in Colorado.

Each plane costs $6,600 per flight hour, according to a U.S. Forest Service report.

On Monday, each tanker made one flight on the Waldo Canyon fire, now estimated at 4,500 acres and 5 percent contained. The fire has not burned any structures, but it is bumping up against the Cedar Heights subdivision. That neighborhood remains under mandatory evacuation, as does Mountain Shadows south of Chuckwagon.

According to Jennifer Jones, a spokeswoman for the service at the National Inter-Agency Fire Center in Boise, Idaho, the heavy military aircraft are not used to fight fires until all other existing resources — including private craft — are unavailable.

“The most important criteria — and the only one that is required to be met — is that commercial tankers are committed or not readily available,” Jones said.

Jones said that was when the National Multi-Agency Coordinating Group, which is composed of federal, state and local officials, decided to bring in the military tankers.

The only other officials who can decide to deploy such C-130s are the governors of California, Wyoming and North Carolina, where Air National Guard Airlift Wings with the special planes are based. But the governor of Colorado, where the only such C-130s are stationed with an Air Force Reserve unit, has no such authority.

Roxane White, chief of staff for Gov. John Hickenlooper, said state emergency-management authorities notified her just before 7 p.m. Saturday that U.S. Forest Service officials said firefighting efforts needed to reach a new stage.

The aircraft were in the air by 1 p.m. Monday, White said.

“We’ve asked repeatedly on the fires, ‘Are we at a stage where we can request these?’ ” White said, adding that firefighting officials said that until Saturday, there were still commercial aircraft available.

Jones said a federal agreement with the U.S. Department of Defense governs the use of the big planes.

“Their role is not to be first responders,” she said. “Their role is to provide us with a surge capability.”

The fires marked the first time any of the aircraft had been used in Colorado this year.

One of the tankers is piloted by reservist Lt. Col. Robert Fairbanks, who has been flying C-130s for 16 years and been on the reserve squad for seven.

“It is more personal being right here in our hometown,” Fairbanks said. “It’s unfortunate that they need us, but we are always happy to help out.”

Incident commander Rich Harvey said good progress was made on the fire Monday.

“Five percent doesn’t sound like a lot,” he said, “but it’s more than zero and we’re going in the right direction.”

Investigators still have no idea how the fire started. They’re seeking any possible leads from the public.

Pike and San Isabel National Forest supervisor Jerri Marr said Waldo Canyon is the No. 1 priority fire in the country.

“It doesn’t mean it’s more important than the High Park fire near Fort Collins, or others burning throughout the West,” Marr said. “It’s an assessment based on the complexity of the fire, its movement in multiple directions, the shifting winds and its proximity to residential areas.”

The owners of Boulder’s Sterling University Peaks apartments, who this summer were cited for illegally subdividing 92 bedrooms in the complex, have reached an agreement to settle the case for $410,000, the city announced Thursday.